Optical sensors based on semiconductors commonly measure absorption/reflectance, scattering, luminescence, and fluorescence. Optical absorption measurements are commonly used for chemical sensors. The sensors usually consist of a Light-Emitting Diode (LED), two optical fibers and a photodiode (PD) or phototransistor. Light from the source is transmitted through the first fiber to a testing chamber filled with a reagent gas. The higher the gas concentration the less light is transmitted by the second optical fiber from the chamber to the photodetector. Rapid development and diversification of optical wave guide devices for the detection of toxic gases and vapors in air and small molecules in aqueous solutions has led to fabrication of practical integrated planar optical chemical sensors. The book Chemical Sensors by Edmonds, pp. 278-282, provides background information on this type of sensor.
Scattermeters and motion control encoders for motors use LEDs or laser sources to generate a beam of light, a surface under test or a patterned cylinder to reflect the beam, and a photodetector (PD) to monitor the variation of the scattered or reflected light. The optical encoders typically employ infrared emitters and detectors operating in the spectral range from 820 to 949 nm. Such components are usually based on p-n junctions formed on conventional semiconductors, such as silicon and gallium arsenide, and are temperature-sensitive, have large size, and require special arrangements to reject the ambient light.
Other optoelectronic sensors measure light-induced fluorescence. Measurement of fluorescence, the absorption of light energy at one wavelength and its emission at a longer wavelength, has become widely accepted as a highly specific, convenient, and sensitive analytical technique. A comparison of analytical techniques indicates that fluorometry is at least 1000-times more sensitive than absorption spectroscopy. Compounds not emitting fluorescent light may be labeled with a fluorescent reagent. Most remote systems have used the fluorescence resulting from excitation at a single wavelength. Greater discrimination can be achieved using multiple or broader optical beam excitation wavelengths. A broader beam can simplify the design and yield a remote monitoring tool capable of detecting a specific target substance in a varying background and indicating its concentration. For biomedical applications, using fluorophores on the ends of fibers can make possible simultaneous measurement of pH, carbon dioxide, and oxygen in a sample. Such a unit has been developed by CDI-3M Health Care, for example. Chemical testing has been demonstrated using fiber-optic fluorimmunoassay (FOFIA). In this technique, antigens specific for the antibodies to be detected are immobilized in proximity to a guided optical beam. The antibodies are tagged with fluorophores and allowed to bond to the antigens. Evanescent excitation of the fluorophores and/or collection of the resulting fluorescent radiation provide for extremely sensitive monitoring techniques.
Temperature can be determined by measuring fluorescence emission decay times from rare-earth-doped and transition-metal-doped phosphors. In another application of fluorescence, flow rates can be measured by dye dilution. Water flow, for example, can be measured by injecting and mixing a dye at a constant rate and using a fluorometer to determine how much the water stream has diluted the dye.
The commercially available optoelectronic device based on fluorescence, a “fluorometer,” contains an excitation source, a sample cell, optical filters and a fluorescence detector. The excitation source is usually a deuterium or xenon lamp. Broadband excitation light from a lamp passes through a monochromator. Fluorescence from the sample is dispersed by another monochromator and detected by a photomultiplier tube. Scanning the excitation monochromator produces the excitation spectrum and scanning the fluorescence monochromator produces the fluorescence spectrum. Modern instruments use only bandpass interference filters to select the excitation and emission wavelengths and LEDs and photodiodes as light sources and detectors.
U.S. Pat. No. 5,822,473 discloses an integrated microchip chemical sensor. An LED light source, a waveguide and a silicon photodetector are placed on the same chip. The amount of light propagating through the wave guide and measured by the detector is affected by a chemical sensitive material coated on top of the waveguide.
U.S. Pat. No. 5,442,169 discloses an integrated optical sensor module on the same substrate using various configurations of a wave guide. The '169 patent requires the integration of at least one optical waveguide in the sensor to measure one or more variables and does not provide a method of sensor fabrication requiring only a single technological process. What is needed is an optoelectronic sensor containing both a light source and a detector that can be fabricated on a chip using a single technological process.